1. Field of the Invention
This invention relates to earth-boring bits of the type using rotatable cutters, especially those having wear pads that enhance rotational stability.
2. Background Information
The earth-boring bit having rotatable cutters or cones is commonly known as the rock bit, even though its use is not limited to those geological formations known as rock. The bit may experience rapid lateral displacements during drilling in an even slightly oversized borehole, a major cause of accelerated wear and catastrophic failure of the cutting elements, which often are called "teeth." Other causes of lateral displacement include doglegs, keyseats, and horizontal drilling, all of which can cause the bit to rotate about an axis other than its intended or designed rotational axis. These lateral displacements cause disruptions from desired rotation about the geometric centerline of the bit, or intended rotational axis. A particularly harmful form of lateral displacement results in reverse rotations or chaotic motions about the rotational axis of the bit called "backward whirl," which can damage the teeth, bearings, and seals. Backward whirl and similar dysfunctions tend to be unstable and worsen over time. In contrast, the teeth of a rotationally stable bit move in generally concentric circles about a stationary rotational axis with minimum slippage relative to the borehole bottom, which reduces wear and inhibits catastrophic failures.
Prior-art rock bits have stabilization pads to reduce lateral movements and create rotational stability. However, the stabilizing pads of these bits are positioned generally with the center of the pad aligned with the rotational axis of each cutter. While such pads are somewhat beneficial in rock bits having cones with positive offset with respect to the rotational axis of the bit, they are not placed sufficiently far from the region of contact between the cutters and the borehole wall to effectively counteract rotation about points of cutter contact on the periphery of the bit and thus effectively minimize or arrest lateral vibrations and backward whirl. Also, with the positioning of the conventional pads, lateral displacements are resisted with the pads being at a substantial angle to, instead of being aligned with, the wall contact forces.